Topic 2: The ecosystem
The ecosystem – a community of interdependent organisms and the physical environment that
they inhabit.
Abiotic and biotic factors
2.1.1 Distinguish between biotic and abiotic (physical) components of an ecosystem.
An ecosystem is a type of system. Like all systems, it has a boundary and it contains a number
of components, which interact with each other.
An ecosystem is made up of:
The living things that it contains
The physical environment it contains
The interactions between the living and non-living components.
Biotic factors
Biotic factors are the living components of an ecosystem.
That is, they are the organisms, or the products of organisms, that directly or indirectly affect
an organism in its environment.
Biotic factors include: living things, the interactions between living things, and the waste
produced by living things.
Abiotic factors
Abiotic factors are the non-living, physical and chemical components of an ecosystem.
Abiotic factors include:
The water, including its chemical and physical properties
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The air, including its chemical and physical properties
The soil, including its structure and chemistry
The climate, including temperature, precipitation and light intensity
Seasonal variations in physical and chemical properties
The presence and levels of pollutants
Limiting factors
Limiting factors – factors which prevent a community, population or organism growing
larger.
Examples:
In most aquatic systems – phosphate is a limiting factor.
In tundra - low temperatures freeze groundwater – water availability becomes a limiting
factor to plants.
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Trophic levels, food chains and food webs
2.1.2 Define the term trophic level.
2.1.3 Identify and explain trophic levels in food chains and food webs selected from the
local environment.
Relevant terms (for example, producers, consumers, decomposers, herbivores, carnivores, top carnivores)
should be applied to local, named examples and other food chains and food webs.
Food chains
A food chain shows the feeding relationships between species in an ecosystem. It thereby
shows the flow of energy from one organism to the next along the chain.
By convention, the species in a food chain are connected by arrows which point in the
direction of the transfer of energy/biomass.
Example:
Sun
→
grass
→
zebra
→
lion
Source of energy for almost all ecosystems: the Sun.
Solar radiation – light.
(Some deep ocean vents – release heat – basis for a small number of ecosystems)
Plants and other photosynthetic organisms capture light energy from the Sun and convert this
to chemical energy in organic compounds:
Process: photosynthesis.
Organisms: producers.
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Some animals eat plants and other photosynthetic organisms.
Process: feeding
Organisms: herbivores.
Some animals eat herbivores.
Process: feeding
Organisms: carnivores
Food chains usually start with a primary producer. Within the consumers there is a hierarchy
of feeding. The carnivore at the end of the food chain is termed the top carnivore.
Trophic level
A trophic level – the position that an organism or a group of organisms in a community
occupies in a food chain.
The trophic levels are named as follows:
Trophic level 1 – producer
Trophic level 2 – primary consumer (herbivore)
Trophic level 3 – secondary consumer (carnivore)
Trophic level 4 – tertiary consumer (carnivore)
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Classification according to energy source
Organisms can be classified according to the manner in which they obtain energy:
Producers / autotrophs. Produce their own food (organic substances) from simple inorganic
substances using the energy of sunlight, or another comparable energy source.
Consumers / heterotrophs. Obtain their food (organic substances) by feeding on autotrophs or
other heterotrophs. Their energy comes from the chemical energy of this food.
Classification of consumers
The consumers include: herbivores, carnivores, omnivores, detritivores (detrivores) and
decomposers.
Herbivores – animals that eat plants
Carnivores – animals that eat other animals
Omnivores – animals that eat both plants and animals
Detritivores – animals that eat detritus or decomposing organic matter (e.g. a dead organism,
faeces, shed skin from a snake).
Decomposers – organisms that obtain the energy from dead organisms. Include many bacteria
and fungi. They obtain their nutrients by secreting enzymes that break down the organic
matter.
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Examples of food chains
Example from a rainforest in north-east Argentina
Organism:
Trophic level:
Jaguar
Tertiary consumer
(Panthera onca)
Top carnivore
Tegu lizard
Secondary consumer
(Tupinambis teguixin)
Carnivore
Heliconius butterfly
Primary consumer
(Heliconius erato)
Herbivore
Passionflower
Producer
Trophic level 4
Trophic level 3
Trophic level 2
Trophic level 1
(Passiflora schummaniana)
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Example from a chalk grassland in Europe
Organism:
Trophic level:
Goshawk
(Accipiter gentilis)
Sparrowhawk
(Accipiter nisus)
Flycatcher
(Muscicapa striata)
Carrot fly
(Psila rosea)
Carrot plant
(Daucus carota)
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Local example
Task: Construct a food chain for a named local ecosystem. In your food chain, name the
organisms involved (the species), arrange them in order of feeding and identify the trophic
levels.
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Food webs
Usually, many organisms in an ecosystem have feeding relationships with each other that are
more complex than can be represented in a single chain.
Within an ecosystem, a number of food chains can be identified. However:
 Some species occur in many food chains.
 Sometimes, very many such single chains would need to be stated to describe the
entire ecosystem.
 Some species feed at more than one trophic level.
The complex network of interrelated food chains in an ecosystem is often better represented
in a food web.
A food web shows the often complex feeding relationships of a number of organisms in an
ecosystem, illustrating this as a network.
Example: food web on the African savanna
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Interpreting food webs
Consider the food web on page 47.
Answer the questions at the top of page 48.
Local example
Task: Construct a food web for a named local ecosystem. In your food web, name the
organisms involved (the species), arrange them according to their feeding relationships and
identify the trophic levels.
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Pyramids of number, biomass and productivity
2.1.4 Explain the principles of pyramids of numbers, pyramids of biomass, and pyramids of
productivity, and construct such pyramids from given data.
Pyramids: graphical models of the quantitative differences that exist between the trophic
levels of a single ecosystem.
In these models:
 Each trophic level is represented as a horizontal bar.
 The bar forming the base is the first trophic level.
 The bar above this is the second trophic level.
 Each successive trophic level is shown as a bar above the previous trophic level.
The bar may represent the number of individuals, the biomass, or the transfer of energy,
depending on the type of pyramid.
Uses: allow us to examine energy transfers and losses.
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Pyramid of numbers
A pyramid of numbers represents the number of individuals at each trophic level in a food
chain.
Relative numbers of individuals shown by the length of the relevant bar.
Usually:
Largest number at bottom.
Each successive bar is successively smaller.
Example for food chain: grass, rabbits, foxes.
Sometimes, the pyramid becomes inverted, or its shape is distorted.
Example for food chain oak tree, caterpillars, blue tits.
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A single tree supports a large number of small herbivores, which in turn support a small
number of carnivores.
A particularly broad bar may be split, to allow differences in the smaller values of the other
bars to be shown more clearly.
Advantages
Simple and easy to construct
Provides an overview
Enables comparisons of changes in population numbers over different times.
Disadvantages
Does not allow for different sizes of different species – inverted or distorted forms.
Difficult to accurately represent very large numbers.
Some animals feed at more than one trophic level.
Pyramid of biomass
Represents the biomass of the organisms at each trophic level.
Biomass – the quantity of dry organic matter of organisms at the respective trophic level.
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Can be measured in units such as:
 grams of biomass per square metre (g m–2).
 or in units of energy, such as J m–2.
Usually pyramid shaped.
Example for food chain of oak tree, caterpillars and blue tits.
Some exceptions – e.g. oceanic ecosystems:
Producers – phytoplankton (unicellular green algae)
Reproduce rapidly – but at any one time are present only in small amounts.
At any one moment in time, the phytoplankton may have a smaller biomass than the primary
consumers, the zooplankton.
Advantages
Overcome problems of organisms with differing sizes distorting the shape of the pyramid.
Disadvantages
Biomass estimated from samples – not possible to know the exact true value
Determining biomass requires killing the organism (drying)
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Time of year greatly affects biomass – e.g. algae vary much throughout year.
The period of time required for the organisms at a given trophic level to acquire their biomass
may be very different. E.g. a large tree may need many years to acquire the same biomass that
algae in a lake may acquire in a few days. This difference is not apparent in a pyramid of
biomass.
Organisms may differ in the amount of energy per mass that they contain. Fats contain about
twice the potential chemical energy per gram than proteins and carbohydrates, so organisms
with much fat have more potential chemical energy per gram than do organisms with less fat.
Some organisms also have much of their mass in forms that are not readily digestible – e.g.
exoskeletons of marine crustaceans.
Pyramid of productivity
A pyramid of productivity represents the flow of energy through each trophic level.
Each trophic level shows by the length of its bar the energy that is produced and made
available as food to the next trophic level during a given period of time.
Always pyramid shaped - unless some disruptive event is taking place.
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Second law of thermodynamics
“In any process, the entropy of the universe must either stay the same or increase”.
During energy transformations, some of the energy is degraded to lower quality and is finally
converted into heat energy, which becomes lost to the surrounding environment.
In accordance with this law, there is a tendency for the amount of useful energy available to
decrease along food chains. Consequently, the pyramids become narrower as one ascends.
Measurement
Productivity is a measure of a rate of flow.
Amount of energy per unit area per period of time
Commonly - joules per square metre per year
Jm-2yr-1
Sometimes - g m–2 yr–1
Usually, only about 10% of the energy at one trophic level is passed on to the next trophic
level. So, each bar is usually about 10% of the length of the one below it.
Supermarket analogy:
The turnover of shops cannot be compared by simply comparing the goods displayed on the
shelves. The rates at which the shelves are being stocked and the goods sold also need to be
known.
Business analogy:
A business may have substantial assets but cash flow may be very limited. In the same way,
pyramids of biomass simply represent the momentary stock, whereas pyramids of
productivity show the rate at which that stock is being generated.
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Important differences:
Pyramids of biomass
Pyramids of productivity
Show a situation at one moment in time
Show a situation over a period of time
Usually pyramid shaped, but much variation
is possible
Always pyramid shaped in a healthy
ecosystem.
Represent storages
Represent flows
Measured in units of mass or energy (for
example, g m–2 or J m–2),
Measured in units of flow (for example,
g m–2 yr–1 or J m–2 yr–1).
Constructing pyramids
Using the information provided in the table on page 51, draw a pyramid of numbers, a
pyramid of biomass and a pyramid of productivity.
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2.1.5 Discuss how the pyramid structure affects the functioning of an ecosystem.
Concentration of non-biodegradable toxins in food chains
Toxins may be present in the environment that do not break down, or which break down very
slowly. E.g. some pesticides, heavy metals.
These toxins may be taken into the bodies of plants and animals – e.g. from water, soil or air.
Animals may also take in these toxins from their food.
In many cases, the toxin may be removed by excretion, egestion or chemical breakdown.
However, in some cases, they accumulate in the bodies of living things over time (termed
bioaccumulation). If the levels of these toxins reach a high enough a level, they may cause
illness or death.
Herbivores eat many plants, or a large amount of plant material, during their lives. They thus
acquire the toxins from many plants (or much plant material). The toxins may accumulate in
the herbivores’ bodies to levels that are much higher than in the plants that they eat.
A carnivore that eats these herbivores will consume many herbivores during its life. It
similarly acquires the toxins from these herbivores, which accumulate in their bodies to levels
that are much higher than in the herbivores that they eat.
With increasing trophic level, the amount of these toxins in the bodies of organisms increases
in this manner. This is termed biomagnification.
The concentrations of these toxins may be too low to cause problems for organisms at lower
trophic levels. However, at higher trophic levels they may be present at levels that cause
considerable problems.
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Pesticides
This effect has been particularly noted with pesticides, especially the early ones, such as
DDT. Did not degrade – still present. Caused noticeable effects – e.g. very thin eggshells for
some birds at high trophic levels. Effects publicized in the book “Silent Spring” by Rachel
Carson – important landmark in the development of the environmental movement.
Modern pesticides – usually degrade over time.
Some pesticides have been developed to decompose into harmless substances on contact with
the soil.
Interpreting information on toxins in a food web
See the example on page 52 (figure 3.31). Answer the questions.
Case studies
Read about the example of mercury in Minamata Bay on page 53.
Read about the example of PBDE on page 54.
What is similar about these two examples?
What is different about these two examples?
What can be done to prevent such events happening?
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Limited length of food chains
Generally – only 10% of the energy in one trophic level is transferred to the next.
We can say – the trophic efficiency is 10%.
Much energy used in respiration – used in operation of the processes of life – becomes lost as
heat to the environment (second law of thermodynamics).
Some material destroyed without being eaten, or not able to be digested.
Some material not eaten – dies and decomposes.
As about 90% of the energy at any one trophic level becomes lost to the trophic level above it,
the amount of energy available to higher trophic levels decreases rapidly.
Very little energy is remaining beyond about four trophic levels for terrestrial ecosystem and
five trophic levels for acquatic ecosystems.
Not usually enough to support viable populations at higher trophic levels.
Small populations of organisms at the higher trophic levels that are present.
Vulnerability of top carnivores
The top carnivores (at the highest trophic level):
- are usually the most vulnerable to changes in the environment.
Especially vulnerable for several reasons:
Biomagnification of toxins in the food chain.
Often have a limited diet. A change lower down in the food chain has a knock-on effect on
them.
Small populations. Less able to withstand disruptions than can organisms with larger
populations.
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Describing ecosystems
2.1.6 Define the terms species, population, habitat, niche, community and ecosystem with
reference to local examples.
Species
A species is a particular type of organism. Individuals belonging to the same species can
interbreed to produce fertile offspring.
Naming:
There is a scientific name for each species. This has two parts: a genus name and a species
name.
Rules:
The genus name comes first, then the species name.
The genus name always starts with a capital letter.
The species name always starts with a lower case letter.
The scientific name is always in italics (if written on a computer) or underlined (if written by
hand)
Local examples include:
Common name
Scientific name
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Population
A population – a group of individuals of the same species living in the same area at the same
time. The members of a population are able to interbreed.
Local examples include:
Habitat
A habitat – the environment in which a species normally lives.
This includes the abiotic environment.
Many populations of different species may share the same habitat.
Local examples include:
Niche
A niche is the way in which an organism makes its living.
The niche includes all the relationships that an organism may have, such as:
Its location
Its response to the resources that are available
Its response to predators
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Its response to competitors
The manner in which it alters such biotic factors
The niche also includes abiotic factors, such as:
The space that is available
The amount of light available
The amount of water available
No two species can inhabit exactly the same niche in the same place and at the same time.
Many species do live together in a community – but they have different niches – i.e. different
needs and responses.
Local examples include:
Community
A community – a group of populations living and interacting with each other in a common
habitat (in the same place).
All the biotic components of a habitat are present in a community.
Local examples include:
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Ecosystem
An ecosystem - a community of interdependent organisms and the abiotic environment that
they inhabit.
Vary in size.
Different ecosystems interact with each other – together form the biosphere.
Local examples include:
The biosphere
The biosphere – the part of the Earth that is inhabited by organisms.
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Population interactions
2.1.7 Describe and explain population interactions using examples of named species.
Populations
A group of organisms of the same species living in the same area at the same time and
capable of interbreeding.
Population density
The average number of individuals in a stated area.
Examples: deer km-2, bacteria mm-3
Factors affecting population size
Four factors:
Natality (birth rate)
Mortality (death rate)
Immigration
Emigration
Competition
The resources in an ecosystem exist in a limited supply. Living things compete to gain access
to these. In their competition to gain access to resources, living things affect each other.
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Types of such resources include:
Intraspecific competition
Competition between members of the same species.
Much affected by the population density.
At low population density:
Generally - every individual has an adequate access to many resources – little competition for
these resources.
Provided adequate access to a mate – high population growth.
Increasing population density:
Increasing level of competition for resources.
Carrying capacity:
The maximum number of a species that an environment can sustainably support.
Sets a limit to the population density.
Competition severe – those best adapted to gain access to limited resources are those who
survive to reproduce – important basis for the concept of evolution by natural selection.
Effect of intraspecific competions:
Tends to stabilise population numbers at about the level of the carrying capacity of the
ecosystem.
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Examples of intraspecific competition:
Seagulls competing for nesting sites on a sea cliff.
Deers competing for food resources by maintaining territories.
Note:
The carrying capacity may change with time.
Example – by means of technology and social organisation, humans have changed the
carrying capacity for their own environment – permit far higher population densities than
previously possible.
Interspecific competition
Competition between individuals of different species for the same resource.
Possible outcomes:
A balance – both species share the same resource
One species may completely outcompete the other - competitive exclusion
Interspecific competition reduces the carrying capacity for each competing species – both
species use the same resource.
Example of interspecific competition
Northern European deciduous woodlands - plants compete for light.
Large trees, such as oak trees and beech trees, often gain dominance by producing higher
canopies. These canopies intercept light, which is no longer accessible to those at lower
levels.
Many herbaceous plants that would be present in an open meadow, such as dandelions and
many grass species, are excluded from the forest floor, or their growth is much limited.
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Some herbaceous plants are abundant in the forest – e.g. snowdrops, primroses and bluebells.
Grow flower and reproduce before tree species have burst into leaf. Avoid direct competition
with the forest canopies by completing the stages of their life cycle that require the most
energy before light becomes greatly restricted by the trees.
When a tree falls, a clearing is created, which becomes a site of strong competition to gain
control of this resource.
Predation
One animal (the predator) eats another animal (the prey).
Examples:
Lion eats zebra
Wolf eats deer.
In this process, the predator kills the prey.
The population of the predator may be limited by availability of the prey - the greater the
number of animals on which it feeds – the greater its supply of food.
The population of the prey may also be limited by this process – the greater the number of
predators, the more individuals of the prey species are killed.
The populations of predator and prey interact:
An increase in the number of prey may permit an increase in the number of predators.
Such an increase in the number of predators may result in a decrease in the number of prey.
This decrease in the number of prey may result in a decrease in the number of predators.
The decrease in the number of predators may then result in an increase in the number of prey.
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A negative feedback mechanism – usually results in a balance being established and
maintained between the populations of predator and prey.
Herbivory
An animal (the herbivore) eating a green plant.
Examples:
Elephants eating leaves from trees and bushes
Rabbits eating grass
Ants eating a leaf.
As with predator-prey relations, the populations of herbivores and the plants that they eat may
interact by means of a negative feedback mechanism.
Many plants are adapted to reduce losses by herbivory – example:
Thorns - roses
Spines - cacti
A stinging mechanism – stinging nettles
Toxic chemicals – poison ivy
Parasitism
A relationship in which one species (the parasite) lives in or on another (the host), gaining its
food from it.
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Parasites do not usually kill their host – but they do often weaken it – less competitive.
High parasite population densities can result in the death of the host – also results in death of
parasites.
Examples:
Vampire bats - feed on the blood of cattle
Tapeworm – in the intestines of many mammals
Mutualism
A relationship between two or more species in which both or all benefit and none suffer.
A form of symbiosis (living together)
Note: other forms of symbiosis include:
Parasitism
Commensalism - one partner gains benefit, the other is not much harmed – e.g. an epiphyte
(such as an orchid or fern) grows on a tree trunk.
Examples:
Lichens
A close association of a fungus and a green alga or cyanobacterium.
The fungus gains by obtaining sugars that the alga or bacterium has produced by
photosynthesis.
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The algae or bacterium gains by obtaining minerals and water, which the fungus absorbs and
transfers to it.
Result – lichens can colonise bare rock surfaces – too dry for algae alone and too lacking in
organic materials for fungi.
Rhizobium and leguminous plants
Leguminous plants – include beans, clover, peas
Nodules in roots – contain bacteria – Rhizobium. Nitrogen fixing.
Absorb nitrogen gas from air (not accessible to plants) and convert this into ammonium
compounds (accessible to plants).
The plants gain – obtain a greater supply of this mineral in an accessible form.
The bacteria gain – obtain sugars from the plant that were produced by photosynthesis.
Result: Enables their survival on mineral-poor soils.
Mycorrhizal fungi and trees
The fungi form a layer around the feeding roots of many trees.
The plant gains – obtains phosphates that the fungal threads have taken from the soil.
The fungi gain – obtain glucose that the tree has produced by photosynthesis.
Sea anemones and clown fish
Clown fish – provide food for the sea anemone – faeces
The anemone – has stinging tentacles – protect the clown fish from predators.
Try questions on page 65.
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Measuring components of the ecosystem
Measuring abiotic components of the system
2.2.1 List the significant abiotic (physical) factors of an ecosystem.
2.2.2 Describe and evaluate methods for measuring at least three abiotic (physical) factors
within an ecosystem.
The abiotic factors in some ecosystems
Abiotic factors - the non-living, physical and chemical components of an ecosystem.
An abiotic factors can act as a limiting factor – if it is limiting the population of a species.
Every ecosystem has a set of abiotic factors which have importance for the living components
of the ecosystem.
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Marine ecosystems
Abiotic
factor
Description
Importance for living things
Salinity
A measure of the salt concentration in
the water. The major salts in marine
ecosystems are sodium chloride,
magnesium sulphate and calcium
sulphate, as well as bisulphates.
Affects the osmotic balance of
living things. A major factor
affecting the distribution of
species – different organisms are
adapted for different levels of
salinity.
pH
A measure of the acidity or alkalinity
of the water. Technically, it is a
measure of the concentration of
hydrogen ions.
Affect the activity of enzymes and
many biochemical processes in the
cells of living things. Different
organisms are adapted to different
pH conditions.
pH 7 is neutral.
Lower values indicate acidity.
Higher values indicate alkalinity.
Temperature A measure of how hot or cold the water
is.
Dissolved
oxygen
Much affects the activity of
enzymes and many biochemical
processes in the cells of living
things. Affects the distribution of
many living things.
A measure of the amount of oxygen gas Required for aerobic respiration.
that is in solution in the water.
Affects the distribution of many
living things.
Wave action A measure of the physical mass
movement of water at its surface
Can cause physical damage to
organisms stationary organisms
living at or near the shore. Can
change the landscape by
associated erosion or deposition.
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Freshwater ecosystems
Abiotic
factor
Description
Importance for living things
Turbidity
Flow
velocity
pH
Temperature
Dissolved
oxygen
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Terrestrial
Abiotic
factor
Description
Importance for living things
Temperature
Light
intensity
Wind speed
Particle size
Slope
Soil moisture
Drainage
Mineral
content
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Investigation
Choose three abiotic factors from one of these ecosystems.
Describe how these abiotic factors may vary within your chosen ecosystem with depth, time
or distance. Include values, with units.
Describe methods for investigating these three abiotic factors in your chosen ecosystem.
Evaluate these methods. State the advantages and disadvantages of these methods, their
precision and suitability in different situations.
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Measuring biotic components of the system
2.3.1 Construct simple keys and use published keys for the identification of organisms.
Identification keys
Need for identification
In describing ecosystems, it is important to be able to name the living species that are present,
before going on to quantify them, or to describe their interactions.
Difficulties in identifying species
In some situations – not so difficult to find the name of a species.
Example: Identifying trees in a forest in Sweden. Relatively few tree species.
It is practical to look at the pictures and read the descriptions in a book of Swedish trees and
find one that resembles the tree.
In some other situations – somewhat harder.
Example: Identifying insect and plant species in a meadow in Sweden. A larger number of
species.
It may be possible to identify a species by looking at pictures and reading descriptions in
books of Swedish meadow plants and insects, though this may be difficult. It could be very
time consuming to find the name for many of the species.
In some situations – very difficult
Example: Identifying insect species in a tropical rain forest. A vast number of species.
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It would be extremely time consuming, because the number of pictures and descriptions
would have to be so large.
Another difficulty is that the differences between species can be subtle and hard to see.
It is thus useful to have a technique for efficiently identifying species.
One such technique is the identification key.
Set up and use
A key consists of a series of questions, each of which has a limited number of alternative
answers. In a paired statement key, there are only two answers for each question.
An answer may be the name of a particular species or a direction to go to another question.
A person using the key answers questions in order, starting with the first and following the
directions given in the answers, until the name of the species is given, or until it is apparent
that the key does not contain the species.
Using a key
Consider the example of a paired statement key used to identify species of aquarium plants.
Constructing a key
Construct a paired statement key for the arthropod species shown on page 312.
Construct a paired statement key for the arthropod species that you identified earlier in the
autumn.
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Features of a good paired statement key
The questions should refer to characteristics that can actually be seen, rather requiring
knowledge of the user.
For example, it is better to ask whether the organism has six legs, than whether it is an insect.
The questions/statements should refer to characteristics that can be seen at the time that the
organism is to be identified.
For example, the question may refer to the number of body parts or legs, but not that the
species usually lays its eggs in the spring.
The question should be easy to interpret, even if only one specimen is available.
For example, a question referring to the number of legs is better than one asking whether the
legs are long.
The two alternative answers to the questions should be clearly distinguishable from each
other.
For example, it is better to ask about the number of body parts, than to distinguish between
two shades of the colour brown.
The questions should first address characteristics that are biologically more fundamental, and
only later more superficial differences.
For example, first ask about the number of legs, later about whether there are hairs on the
legs.
The key should be arranged so that the questions follow each other in an orderly manner, and
so that the structure of the key is as clear as possible.
The key should be able to be extended by adding additional species.
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The key should always lead to the correct identification of a species, if it is present in the key.
They key should preferably not lead to the incorrect identification of a species that is not
present in the key.
Measuring the abundance of organisms
2.3.2 Describe and evaluate methods for estimating abundance of organisms.
Samples
It is not always practical to measure the entire population of an organism. Instead, a small
number are counted, and this is used to estimate the total population. The small number of
individuals that are counted is called a sample. There are different techniques for taking
samples and for using the information that they provide in estimating populations.
A sample
A sample is a part of a population, part of an area, or part of some other whole thing. A
sample of a population would be some of the individuals in a population, but not all of them.
A random sample
In a random sample, every individual in a population has an equal chance of being selected.
The capture–mark–release–recapture method (Lincoln index)
This method can be used on animals that move around.
Example of use:
1. For the population being considered, as many individuals as possible are captured.
This could be by netting, trapping, or searching.
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2. The captured individuals are marked.
3. The marked individuals are released and are allowed to settle back into their habitat.
4. As many individuals as possible are recaptured. The numbers are marked and that are
unmarked are counted.
5. The estimated population size can be calculated using the Lincoln index.
Note:
Animals can be captured by different methods, e.g. netting, trapping or searching.
The animals can be marked in different ways. It is important that the mark does not interfere
with the life of the animal, or make it more likely to be subject to predation.
The Lincoln index:
Population size = n1 x n2
n3
where, n1 = number caught and marked initially
n2 = total number caught on the second occasion
n3 = number of marked individuals recaptured
Example:
In a study of a banded snail population, 50 individuals are captured, marked and released.
Later , 40 individuals from this population are captured, of which 26 are marked and 14 are
unmarked.
Population size = n1 x n2
m2
where,
n1 = number caught and marked initially
n2 = total number caught on the second occasion
m2 = number of marked individuals recaptured
In this case,
n1 = 50
n2 = 40
41
m2 = 26
So,
Population size = 50 x 40
26
= 77
See exercises on page 317.
The quadrat method
This is suitable for estimating plant numbers.
The numbers of plants are counted in small, randomly located parts of the total area. The
sampling areas are usually square and are marked out using frames called quadrats.
One such method:
1. Gridlines are marked out along two edges of the area.
2. Two random numbers are generated. They may be generated by using a calculator
with a random number function, with random number tables, or by some other such
technique. The two random numbers are used as co-ordinates and a quadrat is placed
on the ground with its corner at these co-ordinates. For example, the numbers 8 and 21
may have been generated.
3. The number of individuals present in the quadrat of the plant being studied are
counted.
4. The total size of the area being studied is measured.
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5. The mean number of plants per quadrat is measured. The estimated population size
can then be calculated using the appropriate formula.
Population size = mean number per quadrat x total area
area of each quadrat
Note:
Population density = number of organism per square metre
Percentage frequency = the frequency of occurrence of the organism
Percentage cover = the proportion of ground covered by the organism (e.g. moss, lichens)
Measuring biomass
2.3.3 Describe and evaluate methods for estimating the biomass of trophic levels in a
community.
Biomass – the quantity of dry organic matter of organisms in a given area, in an ecosystem or
at a particular trophic level.
This may include many kinds of organisms: plants, animals, fungi, bacteria, etc.
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The biomass of biological material can be determined by drying it at about 60 – 70 C until it
has reached constant weight.
If this biological material is a sample, the result may be extrapolated to give an estimation of
the total biomass for that species in an ecosystem, or for that trophic level.
Advantages:
Provides a measure of the organic matter content of material, which contains the energy that
is available to other trophic levels. By contrast, with fresh weight, about two-thirds of the
weight often water. This can also vary much between organisms and with time.
A relatively simple procedure for some organisms.
Disadvantages and considerations:
The material must be killed and is destroyed for many purposes in the process. This may be
particularly serious for organisms that are rare (such as tigers) or for which there are ethical
concerns (such as apes). This could potentially involve damage to the habitat.
The material must be located and gathered, which can be difficult for certain components of
the ecosystem, such as animals that move around or plant roots.
For large plants, such as trees and shrubs, it may be difficult to measure an entire individual,
or to accurately assess the proportion of the whole individual that a sample from it represents.
There can be difficult decisions about the amount of each species that should be included in a
sample, and the number of samples needed, in order to give a reliable indication of each
component.
Data of population numbers is often first needed, especially for animals, in order to
appropriately extrapolate the data.
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Measuring diversity
2.3.4 Define the term diversity.
2.3.5 Apply Simpson’s diversity index and outline its significance.
Diversity:
Diversity: a function of two components:
the number of different species and
the relative numbers of individuals of each species.
Diversity is often described by a number - e.g. the Simpson’s diversity index.
D
N(N  1)
 n n  1
You are not required to memorize this formula but you must know the meaning of the
symbols:
D = diversity index
N = total number of organisms of all species found
n = number of individuals of a particular species
D is a measure of species richness.
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A high value of D indicates a highly diverse ecosystem, and suggests a stable and ancient site.
A low value of D indicates an ecosystem lacking diversity and could suggest pollution, recent
colonization or agricultural management.
The index is normally used in studies of vegetation but can also be applied to comparisons of
animal (or even all species) diversity.
Example of use
In a particular sand dune ecosystem, four species were identified, which had the following
numbers:
Species
Number of individuals
Marram grass (Ammophila arenaria)
78
Heather (Calluna vulgaris)
35
Sea buckthorn (Hippophaë rhamnoides)
24
 = 137
N = 137
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D
N(N  1)
 n n  1
D
137(137  1)
78  77   35  34   24  23
D
137(136)
6006  1190  552
D
137(136)
6006  1190  552
D
18632
7748
D  2.40
The Simpson’s diversity index of this ecosystem is thus 2.40.
Answer the questions in the “To do” box, page 318.
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Biomes
2.4.1 Define the term biome.
2.4.2 Explain the distribution, structure and relative productivity of tropical rainforests,
deserts, tundra and any other biome.
Refer to prevailing climate and limiting factors. For example, tropical rainforests are found
close to the equator where there is high insolation and rainfall and where light and
temperature are not limiting. The other biome may be, for example, temperate grassland or a
local example. Limit climate to temperature, precipitation and insolation.
Definition
A biome – a collection of ecosystems sharing common climatic conditions.
These ecosystems generally have similar vegetation patterns.
Types of ecosystems
There are different ways in which biomes are classified.
Example of such a system:
Major type
Subtypes include
Deserts
hot and cold
Forests
tropical rainforest, temperate forests and boreal
Grasslands
savanna and temperate
Tundra
arctic and alpine
Freshwater
swamps, bogs, rivers and lakes
Marine
rocky shores, mud flats, continental shelf and deep ocean
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Examples considered here: tropical rainforest, hot desert, tundra and temperate grassland.
Factors affecting locations of biomes
Major factors are:
climate
terrain - slope, aspect and altitude
Climate
Comprised of:
Temperature
Precipitation - rain and snowfall
Insolation - the solar energy received on an area in a given time
As well as general weather patterns, seasons and weather extremes.
Sun
Major source of energy (light and heat) for living things on Earth
Drives climate systems.
Factors affecting insolation
The atmosphere
Amount and composition of sunlight reaching the Earth’s surface is reduced and altered by
the atmosphere – scattering and absorption - amount affected by haze and cloud cover.
The angle of the Earth’s surface
At the equator, sunlight hits the Earth’s surface at 90.
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With increasing distance from the equator (increasing latitude), sunlight hits the Earth’s
surface at an increasingly more acute angle. The energy is spread over a greater area – less
energy is received per unit area.
Consequently – insolation is generally greatest at the equator and decreases with increasing
latitude (closeness to the poles).
Factors affecting temperature
Latitude - distance from equator.
Affects insolation.
Generally, temperature decreases with increasing latitude.
Altitude - height above sea level
Generally, temperature decreases with increasing altitude.
Highland regions and mountain ranges may have climates comparable with regions
considerably further from the equator.
Winds and ocean currents
These redistribute heat energy from regions close to the equator towards the poles.
Wind: air moving horizontally – from regions of high pressure to regions of low pressure.
Cause ocean currents.
Heat is transferred by water.
Season
The transfer of heat energy by water
This is due to the properties of water:
High latent heat
High heat capacity
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Latent heat
Water can exist as three states:
Solid – ice and snow
Liquid – water
Gas – water vapour
Takes in much energy as it changes from solid to liquid (melts), or liquid to gas (evaporates) –
required to break intermolecular bonds.
Releases much energy as it changes from gas to liquid (condences), or liquid to solid
(freezes).
The energy that is taken in or given out as water changes state is termed latent heat. The
evaporation, condensation, freezing or melting of water in a region can much affect the
temperature.
Heat capacity
Water also has a high heat capacity – much energy is need to raise or lower its temperature.
Water can this be holding much energy – its movement can be associated with the movement
of much heat energy.
Season
The Earth orbits the Sun once per year and rotates once a day about an axis that is tilted at
23.5. During the Northern hemisphere summer, this region is tilted towards the Sun, so the
insolation in this region is greater than at other times of the year. During the Northern
hemisphere winter, this region is tilted away from the Sun, so the insolation in this region is
less than at other times of the year.
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Relationship of precipitation and evaporation
Increasing temperature – causes greater evaporation.
Ratio of precipitation to evaporation: P/E ratio
Important for plants.
Example 1:
150 cm of rain falls in a region in a year and 100 cm are lost by evaporation.
P>E
P/E ratio = 150/100 = 1.5
Example 2:
10 cm of rain falls in a region and 100 cm are lost by evaporation.
P<E
P/E ratio = 10/100
= 0.1
In regions with P/E ratios much greater than 1, rain and snowfall are much greater than
evaporation. Often soluble minerals are washed downwards in the soil – leaching. Can lead to
loss of mineral nutrients from the ecosystem.
In regions with P/E ratios of about 1, precipitation is about equal to evaporation. Soils in these
regions are often fertile.
In regions with P/E ratios much less than 1, water moves upwards through the soil and
evaporates at the surface. Salt may be left behind, leading to an increase in soil salinity
(salinization). This process may prevent plant growth if it progresses far.
Net primary productivity - NPP
Definition
The total gain in energy or biomass per unit area per unit time by plants, after allowing for
losses to respiration.
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Importance of the limiting factor
NPP varies between different biomes – limit in each biome set by the limiting factor – the
factor which is restricting productivity.
Generally because light or a raw material of some sort is in limited supply. Sometimes low
temperature.
Photosynthesis by green plants is the base of most food pyramids – limits in this restrict entire
pyramid.
General trends in net primary productivity with latitude
Near equator:
High, year-round temperatures, high level of insolation and high productivity.
Favourable for photosynthesis.
High values for NPP
With increasing latitude:
Decrease in temperature and level of insolation.
Lower levels of photosynthesis.
Decreasing values for NPP.
Near poles:
Low temperatures, permanently frozen ground (permafrost – water not available to plants),
low isolation, low precipitation (cold air less able to hold water than warm air can).
Low levels of photosynthesis.
Low values for NPP.
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General trends in net primary productivity with level of precipitation
Lack of water – inhibits plant growth – low levels of NPP (despite high temperature and high
insolation).
E.g. in deserts and in semi-arid regions, water is the limiting factor.
Affects also other biomes.
E.g. in temperate regions:
Temperate rainforest – high level of precipitation
Temperate deciduous forest – moderate level of precipitation
Temperate grassland – moderately low level of precipitation
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Biome: tropical rainforest
Hot and wet areas with broadleaved evergreen forest.
Distribution
Around equator – within 5 of latitude North and South of the
equator.
Temperature. High – around 26-28C. Little variation between
seasons.
Prevailing climate
Precipitation. High rainfall – around 2000 – 5000 mm/yr-1. Yearround (no dry season). P > E.
Insolation. High.
Limiting factors
Nutrients may limit plant growth as few nutrients in soil – most in
organic matter. Leaching (P > E, nutrients are washed out of the
soil).
Though isolation is high, the competition between plants for light is
very great. Most intercepted by the canopies –little reaches forest
floor.
Temperature and availability of water are not generally limiting.
Structure
Very high level of biodiversity, with many species and many
individuals of each species.
Competition for light much affects structure. Plants grow tall to have
access to light - form several layers of canopies - stratification.
Includes some very tall trees (emergents), canopies of trees below
these, and some small trees and shrubs, as well as epiphytes that use
large trees as support. Undisturbed forest – few plants on forest floor
(most light intercepted).
Large, broad leaves – better able to intercept light and little need to
conserve water.
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Shallow roots (most nutrients near surface). Some trees have buttress
roots to support them. Very many niches and habitats for animals.
Productivity
Very high level of net productivity. Year-round growing season, with
high levels of photosynthesis, respiration and decomposition.
Younger plants – very rapid growth rate and high biomass gain.
Mature trees – glucose produced in photosynthesis mostly consumed
in respiration. Nutrients are recycled rapidly.
Examples
Amazon rainforest. Rainforest in Congo. Borneo rainforest.
Biome: Desert
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Distribution
Temperature.
Prevailing climate
Precipitation.
Insolation.
Limiting factors
Structure
Productivity
Examples
Biome: Tundra
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Distribution
Temperature.
Prevailing climate
Precipitation.
Insolation.
Limiting factors
Structure
Productivity
Examples
Biome: Temperate grassland
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Distribution
Temperature.
Prevailing climate
Precipitation.
Insolation.
Limiting factors
Structure
Productivity
Examples
International perspective: Biomes usually cross national boundaries (biomes do not stop at a
border; for example, the Sahara, tundra, tropical rainforests).
How do people make use of these biomes?
What issues do people face in their maintenance?
Function
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Ecosystem function - how the ecosystem works.
2.5.1 Explain the role of producers, consumers and decomposers in the ecosystem.
Producers, consumers and decomposers
Producers
Producers are able to synthesise organic compounds from simple, inorganic compounds, using
the energy of sunlight. Also termed autotrophs. They do this by the process of photosynthesis.
For most ecosystems, the producers are green plants.
Role in ecosystem: They act as the base of the food chain.
Consumers
Feed on producers or other consumers. Make use of energy in the complex organic substances
in their bodies.
Also termed heterotrophs.
Include herbivores, carnivores, omnivores, detritivores and decomposers.
Include animals, fungi and most bacteria.
Decomposers
A group of consumers.
Obtain their energy from dead organisms.
Examples: many bacteria and fungi are decomposers.
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Mineral nutrients that are locked up in dead organic matter. In decomposing this dead organic
matter, decomposers make it available to other living things.
Simple inorganic substances and complex organic substances
All matter is made up of atoms. There are about 100 different kinds of atoms.
An element is a substance made of just one kind of atom.
These atoms can be combined in different ways – results in a vast number of different
substances.
Simple inorganic substances
Inorganic substances generally have:
 Few atoms in any one compound
 Relatively low levels of chemical energy
Important examples for life:
Water
H2O
two atoms of hydrogen and one of oxygen
Oxygen gas
O2
two atoms of oxygen
Carbon dioxide
CO2
one atom of carbon and two of oxygen
Nitrate ions
NO3-
one atom of nitrogen and three of oxygen
Nitrite ions
NO2-
one atom of nitrogen and two of oxygen
Ammonium ions
NH4+
one atom of nitrogen and four of hydrogen
Phosphate ions
PO43-
one atom of phosphorus and four of oxygen
Potassium ions
K+
one atom of potassium
Mineral nutrients, such as:
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Complex organic substances
Many are made by living things.
All contain carbon.
Organic substances generally have:
 Very many atoms in any one compound
 Relatively high levels of chemical energy
Important groups for living things:
Carbohydrates, including sugars, starch and cellulose. Contain atoms of carbon, hydrogen and
oxygen.
Proteins. Built up of smaller organic compounds called amino acids. Contain atoms of carbon,
hydrogen, oxygen and nitrogen. Often also sulphur.
Fats and oils. Contain atoms of carbon, hydrogen and oxygen.
Nucleic acids. Contain atoms of carbon, hydrogen, oxygen, nitrogen and phosphorus.
Transformations
Substances can be converted into each other, by rearranging the atoms.
The process of converting one substance to another is called a chemical reaction.
Simple inorganic substances can be converted into organic substances. For this to happen,
energy must be added, because the organic substances contain more chemical energy than do
the inorganic substances from which they are made.
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Organic substances can be converted into inorganic substances. In such a process, energy is
released, because the inorganic substances produced contain less chemical energy than do the
organic substances.
2.5.2 Describe photosynthesis and respiration in terms of inputs, outputs and energy
transformations.
Photosynthesis
Photosynthesis is the process by which green plants and some other living things use the
energy of sunlight to convert simple, inorganic compounds into complex organic compounds.
In this process, the energy of sunlight is converted into chemical energy in the organic
compounds. These organic compounds can then be used to build the bodies of the plants, to
build substances that help the plant to function and as a source of the energy needed so that
the processes of life can take place.
The process of photosynthesis takes place in the green parts of the plant: the leaves and
sometimes the stem. It is carried out in a particular structure inside of these cells called the
chloroplast.
Inputs
The energy of light is required for the process to take place. It is only certain of the visible
wavelengths of light (colours) that provide most of the energy used in photosynthesis
(generally red, yellow and blue).
In the chloroplasts, the green pigment chlorophyll captures the light energy. This is the first
step in the conversion of the light energy into chemical energy. The presence of chlorophyll is
required for photosynthesis to take place.
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The inorganic substances used are carbon dioxide and water.
In most plants, the carbon dioxide comes from the air via small pores in the leaves and stems,
and the water comes from the soil via the roots. For some plants and algae living in lakes and
seas, the carbon dioxide may be dissolved in water. In the process of photosynthesis, the
water is split and combined with the carbon dioxide.
Outputs
The sugar glucose is produced. This can then be converted in other processes into other
organic substances, or used in respiration.
Oxygen gas is released.
Energy transformations
Light energy is transferred into chemical energy in the organic substances produced. The level
of chemical energy is these organic substances is much higher than in the carbon dioxide and
water from which they are made.
The process
Photosynthesis is made up of a large number of chemical reactions. The overall reaction can
be summarised as:
light energy
carbon dioxide + water

glucose + oxygen
chlorophyll
light energy
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6 CO2 + 6 H2O

C6H12O6 + 6 O2
chlorophyll
Respiration
Cell respiration is a process that takes all the time inside the cells of all living things: animals,
plants, fungi, protists and bacteria.
Cell respiration is not the same as breathing, ventilation and gas exchange.
In the process of respiration, complex organic substances are broken down into simpler
substances that have less chemical energy. The energy that is released is converted into other
forms that the cell can use to do useful work and is then ultimately lost as heat. It is the energy
that is released in respiration that enables organisms to carry out the processes of life.
Respiration differs from combustion in that it is made up of many chemical reactions, which
break down organic materials in an ordered manner, so that they energy release can be used
and so that extreme temperatures are avoided.
There are two types of respiration.
Anaerobic respiration. This does not require oxygen gas. Glucose is broken down into simpler
organic compounds and some energy is released, which can then be used to do useful work.
Waste products are formed, which sometimes include carbon dioxide.
Aerobic respiration. This requires oxygen gas. Glucose and other organic substances can be
broken down into the inorganic compounds carbon dioxide and water. Much chemical energy
is released, which can be used to do useful work in the cell.
Aerobic respiration releases very much more energy from organic substances than does
anaerobic respiration. This is why humans require oxygen to live. Breathing, gas exchange
and the circulatory system are important to provide the oxygen need for aerobic respiration
and to remove the carbon dioxide that is produced.
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Aerobic respiration can be represented as:
glucose + oxygen
C6H12O6 + 6 O2
 carbon dioxide + water + energy

6 CO2 + 6 H2O + energy
Compensation point
Plants, like animals and other living things, carry out respiration both day and night.
However, the process of photosynthesis requires light, so plants carry out this process only
during the day.
During the night, oxygen is consumed in respiration and carbon dioxide is produced. No
photosynthesis takes place.
During the day, oxygen is also consumed in respiration and carbon dioxide is also produced in
this process. However, the rate of photosynthesis is so great, that the amount of oxygen
released is far greater than that consumed in respiration, and the amount of carbon dioxide
consumed is greater than that produced in respiration. The plant is a net producer of oxygen
and a net consumer of carbon dioxide.
In early dawn, as the light becomes progressively stronger, the rate of photosynthesis
gradually increases. The consumption of carbon dioxide and the production of oxygen
gradually increase as a consequence. Similarly, in the late afternoon, as light becomes
progressively weaker, the consumption of carbon dioxide and production of oxygen gradually
decrease.
The compensation point is the point at which the amount of oxygen consumed and carbon
dioxide produced in respiration equals the amount of oxygen produced and carbon dioxide
consumed in photosynthesis. It thus occurs in the early morning and the late afternoon.
Biochemical details are not required. Details of chloroplasts, light-dependent and lightindependent reactions, mitochondria, carrier systems, ATP and specific intermediate
biochemicals are not expected.
Photosynthesis should be understood as requiring carbon dioxide, water, chlorophyll and
certain visible wavelengths of light to produce organic matter and oxygen. The transformation
of light energy into the chemical energy of organic matter should be appreciated.
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Respiration should be recognized as requiring organic matter and oxygen to produce carbon
dioxide and water. Without oxygen, carbon dioxide and other waste products are formed.
Energy is released in a form available for use by living organisms, but is ultimately lost as
heat.
2.5.3 Describe and explain the transfer and transformation of energy as it flows through an
ecosystem.
The transfer and transformation of energy as it flows through ecosystems
Some key concepts
Ecological energetics – the study of energy flow and storage in food chains and webs.
Energy flows through ecosystems.
Materials cycle within ecosystems.
First law of thermodynamics – energy is neither created nor destroyed, but may be changed
from one form to another. Termed: the law of conservation of energy.
Second law of thermodynamics – the efficiency of energy conversion to useful work is never
perfect. As energy changes from one form to another, some of this energy becomes
unavailable to do useful work in the system. Mostly, this energy becomes heat, which radiates
out into space.
Solar radiation
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Energy is created in the Sun by a type of nuclear reaction. This energy radiates out from the
Sun in all directions. This energy is of a type called electromagnetic radiation. A part of this is
visible light, and a part consists of forms that are not visible to the human eye, such as
ultraviolet light and infrared light.
Energy leaving the Sun: about 63 million J s-1 m-2 (joules per second per square metre).
The solar constant
Only part of the energy radiating from the Sun reaches the upper part of the atmosphere.
Solar energy reaching top of Earth’s atmosphere: 1 400 J s-1 m-2. Termed Earth’s solar
constant.
The effect of the atmosphere on incoming solar radiation
Of the incoming solar radiation that reaches the top of the atmosphere:
About 60% is intercepted by gases and dust particles in the atmosphere.
Nearly all ultraviolet light is absorbed by the ozone layer
Most of the infrared light is absorbed by components of the atmosphere: carbon dioxide,
clouds and water vapour.
The biosphere depends on the energy that reaches the ground. This is affected by such factors
as:
 Time of day
 Season
 Amount of cloud cover
 Other factors
Solar energy reaching the surface of the Earth
Of the solar energy that reaches the surface of the Earth:
About 35% is reflected back into space – by ice, snow, water and land.
Some is absorbed – heats up the land and seas.
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About 1-4% is available to plants.
Conversion of light energy to chemical energy
This occurs by photosynthesis.
The efficiency of this varies.
Example of wheat – an efficient converter.
Plant absorbs about 40% of energy that hits a leaf.
About 5% reflected.
About 5% passes through.
About 50% not available for biological processes. Outside the range of wavelengths (400 –
700 nm ) that can be used.
Of the energy that is absorbed:
Much is not used in photosynthesis. E.g.:
Absorbed by other components in the cell.
Not a colour used efficiently (blue and red much absorbed by chlorophyll, green
reflected)
Just over 9% of energy hitting a leaf is used in photosynthesis. This is the gross primary
productivity (GPP) of the plant.
The plant uses nearly half of the products of photosynthesis in respiration in order to carry out
the processes of life.
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About 5.5% of the energy hitting a leaf contributes to the addition of biomass, that is the net
primary productivity (NPP).
Efficiency of conversion of energy to food
Low for most ecosystems.
Terrestrial systems – 2-3%
Aquatic systems – 1%. Water absorbs more light before it reaches the plants.
Loss of chemical energy from one trophic level to another
Energy available for consumption by higher trophic levels rapidly declines with increasing
position in the food chain.
This is because of:
Uneaten body parts
Indigestible body parts – faeces
Lost body parts – e.g. antlers
Energy losses due to respiration
Low efficiencies of transfer.
Generally, about 10% of the energy consumed at one trophic level is available to the next
trophic level.
Much affects community structure:
 substantial reductions in bimass with increasing position in the food chain
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 short food chains
Decomposers
Faeces and dead organic matter (matter not eaten by higher level consumers) are used as food
by decomposers.
These organisms use this to build their own bodies and in respiration.
The chemical energy of the substances that make up dead organic matter and faeces thus
becomes the chemical energy of the substances that comprise the bodies of the decomposers,
or is used in respiration.
The use of chemical energy in respiration
By carrying out the process of respiration, organisms can convert the chemical energy of
organic substances into other forms, in order to carry out the processes of life.
All of this energy ultimately becomes heat.
Overall energy conversions by ecosystems
Light energy is converted into chemical energy.
Chemical energy is converted into other forms, and ultimately into heat energy.
The fate of heat energy
Heat energy, whether directly or indirectly absorbed from the Sun, is re-radiated from the
Earth to the atmosphere.
Some is absorbed by gases in the atmosphere, warming the Earth (the greenhouse effect).
Some radiates out into space.
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Activities
Activity 1
Study the diagram on page 68 of the course companion. Answer the questions beside it.
Activity 2
Answer questions 2 – 8 on page 61 of the course companion.
Energy-flow diagrams
Energy flow diagrams can be drawn from studies in ecological energetics.
They show such features as:
the energy entering and leaving each trophic level
loss of energy through respiration
the transfer of energy to the decomposers.
Useful for comparing different ecosystems.
Energy flow diagrams can be drawn in different ways. Important to be able to interpret them.
Important distinction in energy-flow diagrams:
Storages of energy – illustrated by boxes representing the various trophic levels
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Flows of energy (productivity) – often shown as arrows (sometimes of varying widths)
Storages of energy: measured as the amount of energy or biomass per unit area
Flows of energy: measured as rates, e.g. J m–2 day–1
Activities
Activity 1
Study the example of a generalized energy flow diagram through an ecosystem shown on
page 59 of the course companion.
What are the different routes that energy can take through the ecosystem?
In what form does energy enter the ecosystem?
In what form does energy leave the ecosystem?
Activity 2
Study the example of a generalized energy flow diagram through a food web on page 60 of
the course companion.
Answer the questions below it.
Activity 3
Study the example of an energy flow diagram for the Silver Springs community on page 62 of
the course companion.
Answer the questions below it.
Activity 4
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Study the example of an energy flow diagram for an ecosystem shown on page 67 of the
course companion.
Answer question 1 above it.
Biogeochemical cycles
2.5.4 Describe and explain the transfer and transformation of materials as they cycle within
an ecosystem.
Notes from the IBO: Processes involving the transfer and transformation of carbon, nitrogen
and water as they cycle within an ecosystem should be described, and the conversion of
organic and inorganic storage noted where appropriate. Construct and analyse flow diagrams
of these cycles.
Can also be termed nutrient cycles or material cycles.
Materials are absorbed by living things from the soil and from the atmosphere. They move
through the trophic levels and are released back to the ecosystem. Usually via the detritus
food chain. Such cycles are termed biogeochemical cycles.
About 40 elements cycle through ecosystem. Some exist only in trace amounts.
For all such cycles, there are both organic phases (the element is a part of a complex organic
compound in a living organism) and inorganic phases (the element is in a simpler inorganic
compound, outside of living organisms).
The efficiency of movement through the organic phase determines the amount that is
available to living things.
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Generally, the major reservoir for the main elements is outside living things. Inorganic
molecules in soils and rocks.
The flow of substances in the inorganic phase is generally slower than in the organic phase.
The major biogeochemical cycles are those of: water, carbon, nitrogen, sulphur and
phosphorus.
The carbon cycle
Importance of carbon for life
Life is based on carbon – essential element.
Present in all organic compounds – permits the building of large, complex molecules that
contain long chains of carbon atoms. Atoms of other elements are attached to the framework
formed by the carbon atoms.
Locations of carbon
In sedimentary rocks and fossil fuels
In living things
In non-living organic matter, such as in the soil
In the oceans
In the atmosphere
Sedimentary rocks and fossil fuels
Rocks – such as limestone, and chalk
Fossil fuels - fossilized life forms. Coal, oil, natural gas.
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Accounts for most of carbon on Earth.
Living plants and animals
Major component of the bodies of living things.
Non-living organic matter
Detritus. Humus.
The oceans
Carbon is dissolved as carbonate and bicarbonate ions, or present as carbonates in the shells
of marine organisms.
In the atmosphere
As carbon dioxide. About 0.038% of the atmosphere.
Processes in the carbon cycle
Carbon is taken from the atmosphere (as carbon dioxide) and from water systems (as
dissolved forms of carbon dioxide) and fixed in living things (as organic substances) by
photosynthesis. This is done only by producers.
Carbon is released from all living things and returned to the atmosphere (as carbon dioxide)
by respiration.
It is also released from living things by combustion (fires) and returned to the atmosphere as
carbon dioxide.
Carbon (as organic compounds) moves between different trophic levels by feeding.
Carbon (as organic substances) becomes a part of the soil following the death of living things.
It is processed by detritivores and decomposers. Much of this carbon is released to the
atmosphere (as carbon dioxide) by the activities of decomposers.
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Some dead organic matter (organic substances) does not decompose, but instead becomes
transformed by the process of fossilization. It may become peat (organic substances), and
eventually coal (mineral carbon), oil and natural gas (organic substances).
Fossil fuels are burned by combustion, releasing their carbon to the atmosphere (as carbon
dioxide).
Activity
Draw a flow diagram to illustrate the carbon cycle, using the above information.
Now consider the information below.
More processes in the carbon cycle
Some carbon dioxide from the atmosphere dissolves in the waters of the oceans. There is an
equilibrium (balance) between the levels in the atmosphere and those in the oceans.
Some living things in marine environments form shells (calcium carbonate). This may settle
at the bottom of oceans and form sedimentary rocks, such as limestone.
Some of the dissolved forms of carbon dioxide in the waters of the oceans may precipitate
(settle out) and form calcium carbonate. This may also form sedimentary rocks, such as
limestone.
Limestone rocks may undergo weathering, releasing carbon dioxide to the atmosphere.
Activity
Draw a new flow diagram for the carbon cycle, which includes the additional information.
Identify the biotic and abiotic phases in your scheme.
Compare your flow diagrams with those on pages 138 and 84 of the course companion.
In the flow diagram on page 139 of the course companion, values are given for storages (in
gigatonnes of carbon) and flow (in gigatonnes of carbon per year). As appropriate, add these
figures to your own flow diagram.
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The nitrogen cycle
Importance for living things
Essential. A component of proteins and nucleic acids.
Forms and locations
Nitrogen gas
N2
Major component of the atmosphere (78% by volume of dry air)
Not directly available to plants and animals. Some microorganisms can fix it.
Ammonium ions, nitrite ions and nitrate ions
Ammonium ions: NH4+, nitrates: NO3- and nitrites: NO2Present dissolved in soil water and in water systems.
Plants can make use of nitrogen in the form of ammonium ions and nitrate ions.
High levels of nitrite ions are toxic to plants.
In living things
A component of many organic substances.
Processes in the nitrogen cycle
The major processes are:
Nitrogen fixation
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Nitrification
Denitrification
Assimilation
Decomposition
Nitrogen fixation
Processes in which nitrogen gas in the air is converted to a form that is available to plants,
such as ammonium ions.
Nitrogen fixation can occur in a number of ways:
 It can be carried out by nitrogen-fixing bacteria that are free-living in the soil. Type of
bacteria: Azotobacter
 It can be carried out by nitrogen-fixing bacteria that are living symbiotically in the root
nodules of leguminous plants (peas, beans, clover, etc). In this arrangement, the
bacteria receive sugars from the plant and provide the plant with nitrates. Type of
bacteria: Rhizobium
 It can be carried out by cyanobacteria (blue-green bacteria) in soil or water. Important
in rice fields in Asia.
 It can be carried out by lightning. Nitrogen gas is oxidized to nitrates, which are
washed into the soil.
 The Haber process. This is an industrial process by which artificial nitrogen fertilizers
are made. Nitrogen gas is combined with hydrogen gas under pressure, in the presence
of a catalyst. Ammonia is formed.
Nitrification
Nitrifying bacteria: free living bacteria in the soil that live by consuming inorganic nitrogen
compounds.
Some convert ammonium to nitrites. Type of bacteria: Nitrosomonas
Some convert nitrites to nitrates. Type of bacteria: Nitrobacter
Nitrates are readily available to plants.
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Denitrification
Denitrifying bacteria live in waterlogged and anaerobic (low oxygen) conditions.
Convert ammonium, nitrate and nitrite ions to nitrogen gas. This is released to the
atmosphere.
Assimilation
Plants take in nitrates and ammonium by their roots (absorption). The nitrogen is rapidly
incorporated into organic substances (assimilation). Other autotrophs similarly absorb and
assimilate inorganic nitrogen from soil water or water systems, forming organic substances.
Animals and other consumers eat the bodies of other living things. They break down highly
complex nitrogen-containing organic compounds (such as proteins) into simpler organic
compounds (such as amino acids) and then use these simpler organic compounds to build
their own highly complex organic molecules.
Decomposition
Non-living organic matter undergoes decomposition. This includes dead organisms, leaf fall,
egested matter (faeces) and excreted matter (urine).
In this process, organic substances are broken down and ammonium, nitrite and nitrate ions
are produced. These are then available to plants.
Decomposition is carried out by animals (such as earthworms and insects), fungi and bacteria.
More important source of nitrogen in the soil than nitrogen fixation.
Activity
Draw a flow diagram to illustrate the nitrogen cycle, using the above information.
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Compare your flow diagram with those on pages 232 and 85 of the course companion.
The water cycle
Also termed the hydrological cycle.
Consists of storages of water and flows of water between these.
Major storages
Snow and ice (solid)
Groundwater (liquid)
Lakes and rivers (liquid)
Oceans (liquid)
Atmosphere (gas as water vapour, and liquid as water droplets)
Soil (mostly liquid as soil water, may be frozen solid as ice)
Flows of water
Precipitation over oceans
Precipitation over land
Ice melt
Surface run off
Evapotranspiration from land (evaporation and transpiration – evaporation from plants)
Evaporation from sea
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Activities
Draw a flow diagram to illustrate the water cycle, using the above information.
Compare your flow diagram with that of page 217 of the course companion.
Consider the information given in the table on page 217 of the course companion.
Draw a new flow diagram, in which the sizes of the boxes reflect the volumes in the storages
and the widths of the arrows represents the volume of flows.
Definitions of productivity
Production – the making of something
Productivity is the production per unit area per unit time.
We can say that it is amount of something that is made in a given area and in a given time.
We could also say that it is the rate of growth in living things, or the increase in biomass in
living things.
Gross - the total amount of something that has been made.
Net - the amount of something that remains, after deductions have been made.
Primary production – production by autotrophs.
Secondary production – production by consumers.
Units
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Often, productivity is measured as dry mass produced in a unit area per unit time:
E.g.
g/m2/day
g m-2 day-1
g/m2/year
g m-2 yr-1
Productivity is also often measured as energy produced in a unit area per unit time:
E.g.
kJ m
Energy/kJ m-2 yr-1
2.5.5 Define the terms gross productivity, net productivity, primary productivity and
secondary productivity.
Gross productivity
GP
The total gain in energy or biomass per unit area per unit time.
We can say that it is the biomass that could be gained by an organism before any deductions
are made.
Net productivity
NP
The gain in energy or biomass per unit area per unit time after deductions due to respiration
are made.
Primary productivity
Production per unit time by autotrophs (green plants).
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Secondary productivity
Production per unit time by consumers (animals).
2.5.6 Define the terms and calculate the values of both gross primary productivity (GPP) and
net primary productivity (NPP) from given data.
Gross primary productivity
GPP
The total gain in energy or biomass per unit area per unit time by autotrophs.
This is the energy that green plants fix in the process of photosynthesis.
Net primary productivity
NPP
The total gain in energy or biomass per unit area per unit time by autotrophs after allowing for
losses to respiration.
This represents the increase in biomass by the plant. It is also the biomass that is potentially
available to the consumers that eat the plants.
Calculating values of gross and net primary productivity
All the energy fixed by plants in the process of photosynthesis is converted into the chemical
energy of sugars. In theory, the amount of sugar produced should indicate the gross primary
productivity. However, a proportion of these sugars are quickly used in respiration to provide
energy with which to carry out the processes by which the plant sustains itself. It is thus
difficult to measure GPP directly.
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The net primary productivity is the increase in biomass. This can be found by direct
measurement of changes in dry mass.
The gross primary productivity and net primary productivity can be estimated by the
following equations:
NPP = GPP – R
or
GPP = NPP + R
where
GPP is the gross primary productivity
NPP is the net primary productivity
R is respiratory loss
2.5.7 Define the terms and calculate the values of both gross secondary productivity (GSP)
and net secondary productivity (NSP) from given data.
Gross secondary productivity
GSP
The total gain in energy or biomass per unit area per unit time by consumers through
absorption.
The plant material that is eaten by primary consumers (herbivores) represents the theoretical
maximum amount of energy that is available to all consumers in the food chain. However,
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some of the ingested plant material passes through the intestines and is released as faeces.
This process is termed egestion. Food lost as faeces provides no energy to the animal. It is
only the part of the food that passes through the walls of the alimentary canal to enter the
bloodstream that is truly absorbed and which provides the animal with energy. This part of the
energy in the food is termed assimilated food energy. It is this which constitutes the gross
secondary productivity.
Net secondary productivity
NSP
The total gain in energy or biomass per unit area per unit time by consumers after allowing for
losses to respiration.
Animals, like plants, consume a part of their organic material in respiration, in order to be
able to carry out the processes with which they sustain themselves.
Calculating values of gross and net secondary productivity
The following equations can be used:
GSP = food eaten – fecal loss
NSP = GSP – R
where R = respiratory loss
Note:
Most primary consumers assimilate about 40% of the energy in their food and egest about
60%.
Most consumers assimilate about 80% of the energy in their food and egest about 20%.
However, they generally also have to consumer more energy in respiration than do many
primary consumers, and much of their prey is non-digestible material, such as bones and horn.
Exercise
86
Test yourself. Page 67. Question 1
Use the information in the table on page 43 to draw graphs that show the relations between
the following pairs of variables:
Net primary productivity and annual precipitation
Net primary productivity and solar radiation
Describe these relationships.
Distinguish between net primary productivity and mean biomass. Note the units used for each.
Calculate the ratio of plant biomass to animal biomass in the tropical rainforest. Be careful!
Note the units.
Changes in ecosystems
2.6.1 Explain the concepts of limiting factors and carrying capacity in the context of
population growth.
Population growth
Individuals produce more offspring than are required to replace themselves – a key
observation of Darwin.
A population that finds itself in an environment that is favourable to its survival and
reproductive success will tend to increase.
87
Consider the following example
A new species of a bacterium has been discovered. You wish to investigate this bacterium
further. At the moment, you have rather a small sample. In order to produce more of the
bacterium, you place one cell of the bacterium in a container which contains a nutrient-rich
broth of pH 7. You keep the container at 20 C. The bacterium divides by binary fission – one
cell divides to become two. You find that over the course of the following days, the
population doubles at regular intervals.
Table 1. The change in population over time of the bacterium
Time
Population of the bacterium
(hours)
(number of individuals)
0
1
5
2
10
4
15
8
20
16
25
32
30
64
35
128
40
256
45
512
50
1024
This can be plotted as a graph.
Chart 1. The change in population over time of the bacterium
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Population
(number of
individuals)
Time (hours)
In this experiment, there has been no factor limiting the population and it has continued to
grow, doubling at regular intervals. If the trend continues, by a little over four days, there
would be over a million of these bacteria in the container.
Such a population growth is termed exponential growth, or geometric growth.
Exponential growth
Population growth in which the population doubles at regular intervals.
2.6.3 Describe the role of density-dependent and density-independent factors, and internal
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and external factors, in the regulation of populations.
Notes from the IBO: According to theory, density-dependent factors operate as negative
feedback mechanisms leading to stability or regulation of the population.
Limiting factors
Factors that limit or stop population growth.
These are of two types:
 Density-dependent limiting factors
 Density-independent limiting factors
Density-dependent limiting factors:
 Their effect increasing with increasing population size.
 Usually biotic factors.
 Act as negative feedback mechanisms – that is, they restore balance, and lead to
stability or regulation.
Density-dependent limiting factors can be internal factors or external factors.
Internal factors
These are factors that act within a species.
Examples: limited food supply, limited availability of territories, density-dependent fertility.
Internal factors generally introduce a strong competition between individuals for the limiting
resource. In this competition, those individuals who are best able to gain access to and make
use of the limiting resource tend to survive and reproduce, whereas those who are less able to
compete for these resources tend to die without reproducing. This is also a key observation of
Darwin.
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External factors
These are factors that act between different species.
Examples: predation, disease.
Predation: with increased population, easier for predators to find prey, more offspring
produced by predators, both increasing the intensity of predation. In turn, the population of
the prey may decline, leading to a decrease in the population of the predator.
Disease: often population-dependent. More frequent and closer contact between individuals
leads to more opportunities for its spread. Individuals stressed due to lack of nutrition may be
more vulnerable to the disease.
Activity
Read about the example of the Canadian lynx and the snowshoe hare in pages 81 – 82 of the
course companion.
Answer the questions on page 82 of the course companion.
Density-independent limiting factors:
 The effects are not related to population density
 Tend to be abiotic.
Examples:
 The weather – short-term effects, such as a storm
 Climate – long-term weather conditions – e.g. a dry summer
 Volcanic eruptions
 Floods
Carrying capacity
91
This is the maximum number of individuals of a species that a particular environment can
carry or support on a long-term basis.
The carrying capacity is the upper limit of a sustainable population.
It is referred to as K.
2.6.2 Describe and explain S and J population curves.
Notes from the IBO: Explain changes in both numbers and rates of growth in standard S and J
population growth curves. Population curves should be sketched, described, interpreted and
constructed from given data.
Population curves
As a result of the limiting factors, exponential population growth is not sustained. The
changes in population over time can thus better be described by other types of curves, such as
the S-curve and the J-curve.
The S population curve
The S-curve begins with exponential growth.
Above a particular population size, the rate of population increase gradually slows down until
the population is steady.
Example: a small sample of yeast cells are added to a medium that has a constant but limited
supply of nutrients.
The population growth can be described by a series of phases:
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The lag phase
Initially, the population population is small and its rate of growth is slow, as it starts to
multiply.
The exponential phase
The population grows rapidly, doubling at regular intervals. That is, the population is
increasing exponentially, with an ever increasing rate of increase (exponential growth).
Nutrients and other limiting factors are not limiting the population.
The transitional phase
The population continues to increase, but at a slower and slower rate. That is, the rate of
increase declines, until the population is steady.
The competition for nutrients or another limiting factor increases with increasing population.
The stationary phase
The population is at or around the carrying capacity of the environment. It shows no longterm increase or change. It may however show fluctuations about the carrying capacity.
Chart showing an S curve
Environmental resistance
93
The environmental resistance is the area between the exponential growth curve and the Scurve.
It can be any factor that is limiting the increase in the population. Such factors could be lack
of food, lack of space, lack of light, the occurrence of predation or the occurrence of disease.
Activity 1
Consider the following example. A small number of deer have settled on an island on which
there are no other deer and on which there are no predators to them. The population of these
deer were monitored over time.
Changes in the population of deer on the island over time.
Population
(number of
deer)
Time since arrival (years)
Answer the following questions:
What was the population of the deer after 18 years?
94
How long did it take the deer to achieve a population of 25 000?
What is the carrying capacity for this species in the environment? (Include number and units
in your answer).
Describe the changes in the population during the period that the deer were monitored.
What type of curve best describes the above population charge?
What phase in the population curve is the population undergoing on each of the following
occasions:
2 years after the deer arrived on the island.
20 years after the deer arrived on the island.
26 years after the deer arrived on the island
36 years after the deer arrived on the island.
What factors may be limiting the growth of the population from about 25 years after the deer
arrived on the island?
Are these factors density-dependent or non-density dependent?
Are these factors internal or external?
To what extent are these factors operating 16 years after the deer arrived on the island?
The population fluctuates during the period 30 – 40 years after the deer arrived on the island.
What processes may be operating on the population during this phase?
95
Activity 2
Following a period of intensive poaching, the population of jaguars in a game reserve has
been drastically reduced. A new programme is introduced to counter the poaching. The table
below shows the population of the jaguar in the game reserve in the period following the
introduction of the new programme.
The estimated number of jaguar in the game reserve
Period since introduction
of new programme
(years)
Estimated number of
jaguar in the game
reserve
0
83
2
80
4
110
6
156
8
217
10
314
12
425
14
644
16
853
18
1 320
20
1643
22
2 105
24
2 958
26
4 484
28
4 218
30
3 983
32
4 126
Draw a graph showing the changes in population of the jaguar over the period following the
introduction of the new programme.
96
Identify the different phases in the curve.
To what extent has the programme to counter poaching been successful?
What factors may be limiting the population of the jaguars in the stationary phase of the
curve?
Are these factors density-dependent or non-density dependent?
Are these factors internal or external?
The J population curve
The J-curve also begins with exponential growth.
However, the period of exponential growth is followed by a sudden collapse, termed a
dieback.
The population often exceeds the carrying capacity before the collapse occurs – termed an
overshoot.
As the curve does not show the gradual slowdown of population growth with increasing
population size, the population decrease is most likely to be caused by a density-independent
limiting factor.
J-shaped population growth curves are common for microorganisms, invertebrates, fish and
small mammals.
Chart showing a J curve
97
Activity
Consider the following example. An annual flower was introduced to a garden in an area in
Europe that is considerably further north of the usual habitat of the species. The plant spread
to nearby meadows and its population was monitored during the following years.
Population
(number of
individuals)
Time (years)
What factors could be limiting the number of flowers in this example?
Are these limiting factors density-dependent or non-density dependent?
Approximately, what is the carrying capacity of this species in this environment?
98
Situations of different factors operating on a population
Both density-independent and density-dependent limiting factors are often operating on a
population. As a result, the population growth curve usually appears to be a combination of an
S-curve and a J-curve.
2.6.4 Describe the principles associated with survivorship curves including, K- and
r-strategists.
Notes from the IBO:
K- and r-strategists represent idealized categories and many organisms occupy a place on the
continuum.
Students should be familiar with interpreting features of survivorship curves including
logarithmic scales.
K- and r-strategists
Alternative name: K- and r-selected species.
These organisms follow two different reproductive strategies.
The terms K and r come from variables that describe the shape of the population growth
curve.
r
the growth rate of the population
K
the carrying capacity of the environment
The strategies concern the amount of time and energy that species used in raising their
offspring.
K-strategists
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Have a small number of offspring, but invest much time and energy in parental care.
A large proportion of the offspring survive.
Good competitors.
Population sizes usually close to the carrying capacity.
Usually outcompete r-strategists in stable, climax communities.
Examples: large mammals, albatrosses, trees.
The populations of K-strategists are often regulated by density-dependent limiting factors.
r-strategists
Have a large number of offspring, but invest little time and energy in parental care.
A very small proportion of the offspring survive.
Poor competitors.
However, they reproduce rapidly and can make opportunistic use of short-lived resources.
Due to their fast reproductive and growth rates, sometimes exceed the carrying capacity –
population crash results.
Dominant in unstable ecosystems.
Examples: annual plants, fish, invertebrates
The populations of r-strategists are often regulated by density-independent limiting factors, of
which weather is the most important.
Comparison of K-strategists and r-strategists
100
K-strategist
r-strategist
Long-lived
Short-lived
Grow slowly
Grow rapidly
Mature late
Mature early
Offspring: few and large
Offspring: many and small
Give much parental care and protection
Give little parental care and protection
Much invested in individual offspring
Little invested in individual offspring
Adapted to stable environments
Adapted to unstable environments
Appear at later stages in a succession
Appear early in a succession – pioneers and
colonists
Niche specialists
Niche generalists
Intermediate strategies
K and r-strategies are extremes of a continuum of reproductive strategies.
Many species show behaviour that is intermediate.
Survivorship curves
101
Survivorship curves illustrate the fate of a group of individuals of a species. That is, it
indicates the proportion of individuals surviving at each age for a given species or group.
Sketch the survivorship curve on page 164 of the course companion.
The axes:
The horizontal axis: age
The vertical axis: proportion of survivors. This is a logarithmic scale.
In an arithmetic scale, the intervals represent values of 1, 2, 3, 4, 5 …..
In a logarithmic scale, the intervals represent values of 1, 10, 100, 1000, 10 000 ….
The advantage of logarithmic scales is that it is possible to show both very small and very
large values on the same scales.
Curve 1.
This is characteristic for a K-strategist.
Small number of large offspring and parental care leads to small mortality at low age. Most
individuals reach reproductive age and live for most of their lifespan.
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Example: humans
Curve 3.
This is characteristic for an r-strategist.
High mortality in the very early stages of the life cycle of the species.
Example: frogs
Curve 2.
Less common.
Species that have another chance of dying at any age.
Example: some species of birds.
Activity
Answer the questions on page 164 of the course companion.
Succession
103
2.6.5 Describe the concept and processes of succession in a named habitat.
Notes from the IBO:
Students should study named examples of organisms from a pioneer community, seral stages
and climax community.
The concept of succession, occurring over time, should be carefully distinguished from the
concept of zonation, which refers to a spatial pattern.
Key concepts in succession
Succession
The change in species composition in an ecosystem over time.
Succession is directional, with one community replacing another. It is usually initiated by an
event in which new land is created, or existing land is cleared of its living things.
During the course of a succession, the actions of the community change the environment in
such a way as to bring about its replacement by another community.
Primary succession
A succession that begins with a lifeless abiotic substrate.
During the course of the succession, the substrate is gradually colonized by living things and
the succession passes through several stages.
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A sere
A set of communities that succeed one another over the course of succession at a particular
location.
Pioneer community
The first stage in a succession developing on a lifeless, exposed site. Pioneer plant and animal
species colonise the area.
Examples: new land may be created or uncovered at a river delta, a volcanic lava field, a sand
dune or a glacial deposit.
Seral stages
The stages through which a succession passes as it progresses.
Following the stage of colonization, this may include the stages of establishment, competition
and stabilization, before the final seral climax.
Climax community
The final stage in a succession.
Examples of seres
Lithosere – a bare rocky area of ground is colonized, with the succession leading to a
woodland community.
Hydrosere – a lake is filled in by vegetation, with the succession leading to a woodland
community
(Xerosere – primary succession starting on dry land)
Psammosere – a coastal area that is extending by deposition of sand becomes colonized by
plants, with the successioin leading to a woodland community.
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Plagiosere – a natural succession is arrested by human activity, preventing the attainment of
the climax or subclimax community.
Secondary succession
A succession that is initiated by the sudden destruction of an established community, such as
a fire, flood or human activity (e.g. ploughing). The soils are already present – these can
accept wind-borne seeds, or there may be seeds surviving in the soil. The number of seral
stages is thus reduced.
Primary succession
106
Generalised scheme for the stages in primary succession:
Bare, inorganic
surface
The environment becomes available for colonisation. It is devoid of life
and offers extreme conditions. No soil – just mineral particles. Nutrientpoor. Poor water-holding capacity. Temperature extremes common.
Seral stage 1
Pioneer plant and animal plants colonise the area. These are generally rstrategists, with small size, short life cycles, rapid growth and the
production of many seeds or offspring. Windblown dust and mineral
particles form a simple soil, which these plants help retain.
Colonisation
Seral stage 2
Establishment
Seral stage 3
Competition
Seral stage 4
Stabilisation
Seral climax
Increase in species diversity. Invertebrate species become established in
the soil, increasing its humus content and water-holding capacity. The
soil is further enriched by nutrients from the weathering of the rocks.
These changed conditions permit the establishment of new species.
New species lead to further changes in microclimates. The presence of
larger plants leads to less extreme conditions of temperature, sun and
wind. K-strategists can become established and outcompete earlier rstrategists for space, light and nutrients.
Later colonisers become established, restricting the colonisation by even
newer species and displacing the earlier species (such as by shading them
out). Simple food chains have developed into complex food webs. The Kspecialists are usually specialists, large and slow growing (less
productive), with long life cycles and delayed reproduction.
A stable and self-perpetuating community develops. State of dynamic
equilibrium.
Examples of primary succession
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Stages in a primary succession – case study: Sand dunes on the coast of the UK.
Bare, inorganic
surface
Bare sand. Windy. Little fresh water. Nutrient deficient. Affected by
seawater (high salt levels).
Pioneer stage
Example species: sand couch (Elytrigia juncea) - a grass and lyme-grass
(Elymus arenarius).
Salt tolerant. Adaptations to reduce water loss by transpiration: waxy
coatings on leaves, leaves often rolled.
Affect on ecosystem: begin to stabilize new dunes – network of root
systems. By binding sand in one place - form a wind break – encourages
more sand to be deposited – dune become higher. Rapid collection of
sand (> 30 cm per year) can lead to smothering of the plants. Under
such conditions, they are outcompeted by marram grass.
Yellow (white)
dune stage
Example species: marram grass (Ammophila arenaria)
Less salt tolerant – so cannot survive in earliest stage of dune formation.
But – can grow rapidly and so survive in rapidly growing dunes.
Adapted by windy, dry conditions – leaves have a waxy cuticle and can
roll up in dry conditions (reduces transpiration and water loss).
Incorporate silica into cell structure – gives leaves greater strength and
stability.
Effect on ecosystem: stabilize sand a considerable depth. Deep vertical
root system and extensive horizontal root network.
Other examples: sand sedge (Carex arenaria), sea holly (Eryngium
maritimum) and sea bindweed (Calystegia soldanella). Bind sand,
because of their cover of the bare sand and because of their near surface
horizontal network of roots.
Humus accumulation takes place.
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r-strategists become established, such as dandelions, thistles and
groundsels. Can grow rapidly, produce flowers and set seed in a short
period.
Sand stops being deposited - marram grass dies out.
Grey dune stage
The dunes have a nearly continuous plant cover. Dew in autumn and
winter provides some water. Humus accumulation from dead plant
material permits greater water retention.
Events depend on the pH.
If calcareous (i.e. alkaline pH – due to calcium carbonate from shells)
Lichen species (such as Peltigera, Cladonia)) and moss species (such as
Bryum and sand-dune screw-moss, (Tortula ruraliformis) may colonise
the dunes.
Some species that are characteristic of lime-rich areas or of wasteland
close to the sea, may also become established, e.g. viper's-bugloss
(Echium vulgare), evening-primroses (Oenanthera sp.) and wild thyme
(Thymus drucei).
Invertebrate species become more common, including grasshoppers,
caterpillars, bees and spiders.
If acidic (no shell material, or if rainwater has leached out nutrients
from a calcareous dune).
Acidic grassland plants dominate, including such species as gorse
(Ulex), bracken (Pteridium aquilinum), heather (Calluna vulgaris) and
wood sage (Teucrium scorodonia).
Effect of living species: further humus accumulation and development
of a poor sandy soil.
Dune scrub stage
The sandy soil can support pasture grasses and bushes. Such species
include hawthorn (Crataegus monogyna), elder (Sambucus), brambles
(Rubus sp.) and sea buckthorn (Hippophae rhamnoides). Sea buckthorn
has nitrogen-fixing root nodules, which are highly advantageous in this
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nutrient-poor environment.
With the development of the scrub, the shorter species become shaded
out.
Soil development continues.
Hollows between dunes – termed “slacks”. May be wet for much of year
– closer to underground water table. Develop a characteristic vegetation,
such as creeping willow (Salix repens), grasses and herbaceous plants.
Forest
establishment
Pine forest becomes established. These trees have waxy, needle-like
leaves that reduce transpiration and so reduce water loss. They outshade
many other species.
Seral climax
An temperate deciduous forest develops, with oak and ash – the climatic
climax vegetation (CCV) for much of Britain.
Overall trends:
Increases in:
 Vegetation cover
 Soil depth
 Soil humus content
 Soil acidity
 Soil moisture content
 Sand stability
Zonation
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Zonation refers to a spatial pattern. It describes the manner in which an ecosystem changes
along a gradient in an abiotic factor.
Examples of abiotic factors: water content in soil, altitude, salt content.
Examples of locations: vegetation changes on mountain slopes, vegetation changes on coasts.
Zonation is static – it does not change with time.
Succession refers to a change in an ecosystem with time.
Sometimes, a site may show both zonation and succession, such as in some sand dune
environments.
Activities
Activity 1
Read about primary succession following glacial retreat on pages 266 – 267 of the course
companion.
Activity 2
Describe the processes of succession in a hydrosere. You may find useful information at the
following site:
http://www.countrysideinfo.co.uk/successn/
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2.6.6 Explain the changes in energy flow, gross and net productivity, diversity and mineral
cycling in different stages of succession.
Notes from the IBO:
In early stages, gross productivity is low due to the initial conditions and low density of
producers. The proportion of energy lost through community respiration is relatively low too,
so net productivity is high, that is, the system is growing and biomass is accumulating.
In later stages, with an increased consumer community, gross productivity may be high in a
climax community. However, this is balanced by respiration, so net productivity approaches
zero and the production:respiration (P:R) ratio approaches one.
Changes in important measures of the ecosystem during succession
During a succession there are changes in energy flow, gross productivity, net productivity,
diversity and mineral cycling.
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Measure
Early stages
Intermediate stages
Gross
productivity
Low
Increases to a high level
This is due to:
Initial conditions
(abiotic factors) are not
favourable for plant
growth – e.g. lack of
soil, lack of water, lack
of accessible minerals,
etc, depending on the
circumstances.
Later stages
High - often at its
highest level, but it
may also be slightly
This is due to:
lower than in some
Conditions are becoming intermediate stages. In
more favourable for plant an aging forest, older
growth – e.g. deeper soil trees may become less
providing a better access photosynthetically
efficient.
to water and minerals,
etc.
Increased density of
producers
Low density of
producers
Net
productivity
High
This is due to:
Proportion of energy
lost through community
respiration is low - little
biomass is present to be
carrying out the
process.
Initially increases, due to
the increase in gross
primary productivity.
However, respiration is
also increasing, as the
amount of existing
biomass increases.
production:respiration
(P:R) ratio is high (>1)
production:respiration
(P:R) ratio is falling and
so less biomass is being
accumulated.
High net productivity –
i.e. biomass is
accumulating and the
system is growing.
The net productivity
declines as the
succession progresses
towards the later stages.
Declines and may
approach zero.
Gross productivity
levels may be high, but
respiration is also high
(large biomass in the
ecosystem). In mature
forests: slow tree
growth, tree canopy
restricts, more biomass
to roots, much
respiration being
carried out by
decomposers.
production:respiration
(P:R) ratio near 1.
The system is not
growing or
accumulating more
biomass.
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Measure
Early stages
Intermediate stages
Later stages
Energy flow
Relatively simple –
comprises simple food
chains.
More complex.
Energy flow is
generally complex,
with complex food
webs, long food chains
and a large biomass of
consumers.
Few producers (low
primary productivity)
support a short food
chain with a small
biomass of consumers.
Diversity
Complex food webs
develop, with longer
food chains and a greater
biomass of consumers.
This is supported by high
levels of primary
productivity.
Low
High
Relatively few species
are adapted to the
particular abiotic
conditions.
Abiotic conditions that
permit a large bimass in
the ecosystem.
Few niches.
r-strategists often
dominate – can rapidly
exploit new habitats,
short life-cycles.
Many niches that are
exploited by different
species.
K-strategists become
more dominate – good
competitors.
High, but may decrease
slightly in a stable
climax community.
K-strategists dominate
– r-strategists have
been largely
outcompeted.
In a mature forest, the
canopy may restrict
ground level plants.
A balance is established
between the
opportunities for new
species to become
established, opportunities
for existing species to
expand their range and
local extinction.
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Measure
Early stages
Intermediate stages
Later stages
Mineral cycling
Slow.
High and increasing.
High.
Relatively small
biomass of living
things – little dead
material to
decompose and little
uptake of minerals by
plants.
The development of a
large biomass of
living things and of a
deep, humus rich
soil. Much activity
by decomposers,
plants and animals.
Large biomass. Deep,
humus rich soil.
Much activity by
living things,
including
decomposers, plants
and animals.
Large and small
organisms present.
Large organisms
dominate. Some
small organisms
present. Stratified
ecosystem.
Little soil, humus and
mineral content.
Size of organisms
Organisms are of
small size.
Small biomass.
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2.6.7 Describe factors affecting the nature of climax communities.
Notes from the IBO: Climatic and edaphic factors determine the nature of a climax
community. Human factors frequently affect this process through, for example, fire,
agriculture, grazing and/or habitat destruction.
Factors affecting the nature of climax communities
Final seral stage. Termed: climatic climax community
Stable and self-perpetuating.
State of dynamic equilibrium.
Seral climax – the maximum possible development that a community can reach under
prevailing environmental conditions.
The major environmental factors determining the nature of climax communities are:
Climatic factors – e.g. temperature, light and rainfall.
Edaphic factors – the soil.
Examples of the effects of climatic factors:
Determining the biome to which the ecosystem belongs. In central and northern Sweden, the
climax community is of coniferous forests dominated by pine and spruce, with some birch,
whereas in southern Britain the climax community is of deciduous forests dominated by such
species as oak and ash.
Regional effects. In a region in which rainfall is high, a hydrosere sere may end with the
establishment of a raised bog, instead of a forest.
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Examples of edaphic factors
Soils derived from chalk, limestone and serpentine have plant communities that are distinctive
for each of these soil types and which may differ greatly from other soil types.
Disturbance
Communities are often affected by periods of disturbance.
Examples of natural hazards: flood, fire, landslides, earthquakes, hurricanes.
Small scale example: a large tree falls. This leaves a gap in the canopy which is exploited by
pioneer species in the surrounding community.
Disturbances can cause a community to revert to an earlier seral stage, or create gaps in a
community that regenerate, increasing the productivity and diversity of the community.
Human factors
These frequently affect the development of climax communities, through, for example, fire,
agriculture, grazing and/or habitat destruction.
Arrested successions
A sere may be kept at a seral stage by an environmental factor. This could be an abiotic factor
(such as waterlogging) or a biotic factor (such as heavy grazing). This may result in an
arrested community – termed a subclimax community. This can continue developing towards
the true climax community only if the limiting factor is removed.
A stable, old ecosystem is not always a climax community. Example: lodgepole pine forest in
Yellowstone National Park in the USA. Frequent forest fires interrupt succession.
Deflected successions
A deflected community is a climax community that is affected by a natural event or human
activity that much modifies the community.
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Such a community is termed a plagioclimax community.
Examples: pasture, arable farmland and plantations.
Generally have reduced biodiversity.
If human activities cease, the plagioclimax community develops into the climatic climax
community. An example of secondary succession.
Human requirements in agriculture
Human requirements in agriculture conflict with natural succession.
Natural systems lead to: greater complexity, longer food chains, greater diversity, more
biomass and a stratified ecosystem. Net productivity declines as succession progresses
towards the climax community, as the P:R ratio falls to 1.
Maximum yields in food production are favoured by:
 a simple system (a monoculture) in which weed plants (competitors) are excluded, as
are consumers (such as insects)
 the crop is not permited to progress to a climax community
The community remains in a state in which respiration losses are relatively low and so net
primary productivity is high.
Natural systems can have other values:
 Balance in the carbon cycle
 Nutrient cycling
 Climate buffer of forests and oceans
 Esthetic servives
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Measuring changes in ecosystems
2.7.1 Describe and evaluate methods for measuring changes in abiotic and biotic components
of an ecosystem along an environmental gradient.
Measuring changes along an environmental gradient
Changes along an environmental gradient
An environmental gradient:
 A trend in one or more component of an ecosystem.
 Can be abiotic and/or abiotic components.
Examples of situations:
 Shores of lakes, streams and seas
 Forest edges
Main technique – transect.
Description of methods
Line transect
 Simplest type.
 A string or measuring tape is laid out in the direction of the environmental gradient.
 All the species touching the strong or tape are recorded and counted.
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 Measurements of abiotic factors can be made at intervals along the transect.
 Often – data from many line transects is combined to provide sufficient data.
Belt transect
 A strip of chosen width
 Formed by laying two parallel line transects separated by a suitable distance, e.g. 0.5
or 1 m apart.
 All individuals are sampled between them.
 Measurements of abiotic factors may similarly be made at intervals along the transect.
A continuous transect – a line or belt transect in which the whole line or belt is sample
An interrupted transect – samples are taken at points along the line or belt transect (usually
regular horizontal or vertical intervals)
Evaluation of methods
Common advantages:
 Convenient for distances of some metres.
 Quantitative
 Rapid
 Low cost
 Suitable for observing changes in species composition and abundance of plants,
immobile animals and slow-moving animals.
 With appropriate sampling, can reveal changes in abiotic factors.
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Common limitations:
 Not so suitable for animals that move much.
 Laying the transect and sampling may result in trampling, or other types of
environmental damage.
 Collection of materials for abiotic factors may entail environmental destruction.
 Must decide where to place the transect – placement may be subject to bias.
String method
 Most rapid for taking a single measurement.
 Many measurements must be taken to obtain a useful sample.
Belt method
 Less rapid than the string method.
 Obtains results that are based on a larger sample – more reliable.
Continuous transect
 The complete gradient is included
 Boundaries are exactly determined
 May be time consuming, especially for longer transects
 Sampling may in some cases involve environmental destruction.
Interrupted transect
 More rapid than continuous transects, especially for longer transects
 Sampling made at intervals may entail less environmental destruction
 Important stages and exact boundaries may be missed.
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2.7.2 Describe and evaluate methods for measuring changes in abiotic and biotic components
of an ecosystem due to a specific human activity.
Methods and changes should be selected appropriately for the human activity chosen. Suitable
human impacts for study might include toxins from mining activity, landfills, eutrophication,
effluent, oil spills and overexploitation. This could include repeated measurements on the
ground, satellite images and maps.
Measuring changes in ecosystems due to human activity.
Example: Assessing the effects of intensive agriculture on a lake
Intensive farming:
 High yields
 Heavy use of machinery, pesticides and fertilizer
Impact of pesticides:
 May enter groundwater, streams and lakes
 Decrease biodiversity
Impact of fertilizers:
 Include ions such as nitrate, ammonium and phosphates
 Can enter groundwater and streams
 Large amounts can result in eutrophication (very high nutrient levels)
 Stimulate increase in growth of plants and algae – increases turbidity in water
 Less light penetration – algae in deeper water die.
 Decomposition increases – oxygen level decreases
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 Lower oxygen levels – cannot sustain many animal species (e.g. many fish and
crustaceans), which die
Considerations regarding sampling
Measurements should be made of water bodies that are both upstream and downstream of the
intensively-farmed fields. Conditions before and after encountering the fields can thus be
compared. Alternatively, a lake downstream of fields being intensively farmed could be
compared with a lake that is not downstream of land being intensively farmed.
Measurements made on different occasions should take into account such variables as
changes in factors during the course of a day, between days and between seasons.
Observations should be made at intervals that are appropriate for the factor being observed.
For example, a water system could be studied before the surrounding land is put to use in
intensive farming, and in the years following the introduction of this farming method.
The number of samples should be considered. The greater the number of samples, the more
reliable the result, but the greater the amount of time and resources required.
Measurement of oxygen concentration
Oxygen concentrations can be determined with an oxygen probe or by chemical titration (the
Winkler method).
Oxygen probes:
 Rapid
 Quantitative
 Can be carried out in the field
 Requires electronic equipment
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Chemical titration:
 Does not require electronic equipment
 Quantitative
 More time consuming than using a probe and requires greater manipulative skills.
 Useful for checking instrument calibration.
Measurement of turbidity
The cloudiness of a body of water.
Can be measured with optical instruments, or with a Secchi disk.
Secchi disc – a white (or white and black) disk that is attached to a graduated rope.
The disk is lowered until it is no longer visible and the depth is taken. It is then raised until it
becomes visible again. The average depth is calculated – termed the Secchi depth.
 Quantitative, rapid and low cost
 Standardised procedures should be followed
 Result may be affected by different levels of sunlight
Measurement of pesticides
Sophisticated laboratory methods are required – e.g. HPLC (high-performance liquid
chromatography) and GC-MS (gas chromatography – mass spectrometry)
Advantages:
 Very sensitive – can measure substances that are present at very low levels
 Quantitative – give exact values for levels
 Can identify exact chemical compositions even in mixtures in substances
Disadvantages:
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 Time consuming, which limits the number of samples that can be processed
 Expensive equipment required which is bulky and laboratory-based
 A high level of technical training is required to operate the equipment
Measurement of fertilizers
Test kits can be used for nitrate, ammonium and phosphate ions:
 Quantitative
 Relatively low cost and simple to operate
 Can be used in the field
 Require some manipulative skills to be accurate
Probes can be used for nitrates and ammonium ions:
 Quantitative
 Rapid
 Relatively low cost
 Can be used in the field
 Require calibration
Measurement of biodiversity and population changes
Species identities and populations can be identified by:
 Large water plants and larger animals can be identified by direct observation, the use
of identification keys and counting. Collecting equipment, such as nets, can be used as
appropriate.
 Microscopic organisms can be identified by examining samples with suitable tools,
such as a light microscope (for algae and zooplankton), and culture plates (for
bacteria). These species can then be identified with the help of identification keys
 Population numbers for algae and zooplankton can be estimated using a counting
chamber. Population numbers for bacteria can be estimated by the numbers of colonies
formed.
Biodiversity can be quantified using Simpson’s diversity index
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Advantages:
 Relatively rapid
 Relatively low cost
 Provides quantitative values
Disadvantages:
 Larger animal species, such as fish, may be difficult to observe and are mobile.
 Microscopes are difficult to use in the field and so samples need to be brought to the
laboratory for analysis. Cultivation of bacterial plates requires more extensive
laboratory equipment and training, such as the ability to work under sterile conditions.
Time is also required for the growth of bacterial colonies.
 Many species can be difficult to distinguish and identify, even with the use of an
identification key. This is particularly to case for algal species and bacterial species.
 Estimates of numbers are based on samples and may be of limited accuracy.
Activity
Read and make notes on the effects of sewage treatment on water quality (see page 313 of the
course companion).
Answer the questions on page 314.
Measuring the productivity of ecosystems
Read and make notes on determining the productivity of aquatic and terrestrial ecosystems
(see page 319 of the course companion).
Answer the questions of page 320.
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Environmental impact assessments
2.7.3 Describe and evaluate the use of environmental impact assessments (EIAs).
Students should have the opportunity to see an actual EIA study. They should realize that an
EIA involves production of a baseline study before any environmental development,
assessment of possible impacts, and monitoring of change during and after the development.
Definition
Environmental impact assessment – EIA - report made before a development project to
change the use of land.
An EIA involves:
 The production of a baseline study before any environmental development
 Assessment of possible impacts
 Monitoring of change during and after the development
Examples of such projects:
 Converting pastureland into a golf course.
 Building a motorway across a section of countryside.
 Building a wind farm in a coastal area
Content
The report considers the relative advantages and disadvantages of the project.
For this purpose, it establishes the ways in which the abiotic and biotic factors in the
environment would be changed by the development scheme.
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Important tasks:
Identifying impacts – scoping
Predicting the scale of potential impacts
Limiting the effect of impacts to acceptable limits – mitigation
So that the general public can understand the issues, a non-technical summary is included.
Factors considered
An EIA attempts to quantify changes to such factors as:
 microclimate
 biodiversity
 scenic and amenity values
Baseline study
The study of the original status of the environment in the area, before the development work
of the project is started. This provides a base reference against which changes due to
implementation of the project are measured.
EIAs deal with questions about the likely effects of a development project on the natural
environment.
They can also consider the likely effects on human populations – e.g. the effect on human
health or the economic effects on the community.
Uses of EIAs
Usually part of the planning process required by governments when large developments are
considered.
Provide a documented manner in which to examine environmental impacts
Can be used as evidence in the decision-making process of a new development.
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Those developments that required an EIA vary between countries. Generally include:
 New road networks
 Airport and port developments
 Building power stations
 Building dams and reservoirs
 Quarrying
 Large-scale housing projects
Origin of EIAs
1969 – US Federal government – National Environmental Policy act (NEPA)
A priority for Federal agencies to consider the natural environment in any land use planning.
The natural environment thereby received the same status as economic priorities.
Now widely adopted around the world.
In the US:
Environmental assessments (EA) are carried out to determine if an EIA needs to be
undertaken.
An EIA is often called an EIS – an environmental assessment statement.
Evaluation
An EIA is a model of the system being studied. As a model, it is only as good as the
parameters that it uses. It is thus important to choose the right parameters and to have a
reliable values of these.
Changes in land use bring about both gains and losses from different perspectives. These must
be considered and weighed against each other. One such approach is a cost-benefit analysis.
For this, a monetary value is given to all parameters, which can then be compared using the
same unit of measure. However, this still leaves decisions concerning the monetary value to
be given to these parameters.
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